PROBING MISCIBILITY AND INTERMOLECULAR INTERACTIONS IN SOLID POLYMER BLENDS USING THE NUCLEAR OVERHAUSER EFFECT

We discuss a new strategy for probing polymer-polymer miscibility and detecting specific intermolecular interactions in polymer blends by exploiting nuclear Overhauser effects (NOE) in C-13 magic-angle spinning (MAS) NMR spectra of solid blends. In the comparison of deuterated polystyrene (PS)/poly(vinyl methyl ether) (PVME) blends cast from toluene versus chloroform, steady-state equilibrium NOE enhancements of the PS signals were observed for the toluene-cast films but not for the chloroform-cast films. Since cross-relaxation of the PS carbons may occur only through dipolar interactions with PVME protons (which have an inverse sixth power dependence on the internuclear distances), this demonstrates molecular level miscibility for the blends cast from toluene only. Below T(g), methyl group rotation about the C3 axis is the only motion fast enough to stimulate dipolar cross-relaxation in the blend. Additional information concerning the molecular structure of the blend was obtained from the NOE growth rates for each type of carbon nucleus in the blend. The cross-relaxation rate constants for the PS aromatic ring carbons were several times greater than that of the PS main-chain carbons. This implies that the phenyl ring is much closer to the methyl group of PVME than to the main-chain carbons of PS. These results suggest that there exists a specific intermolecular interaction between the PS phenyl ring and the PVME methyl group in the solid state. Such an interaction has previously been suggested for polymer blend solutions.